Virulence heterogeneity among bacterial or viral strains or isolates remains as one major challenge in vaccine development. Like other infectious pathogens, enterotoxigenic Escherichia coli (ETEC) strains are genetically and immunologically heterogeneous. ETEC strains are the most common bacteria causing diarrhea, a disease continues to be the second leading cause of death to children younger than 5 years in developing countries and remains as a major threat to global health. These ETEC strains produce immunologically different colonization factor antigens (CFAs) to colonize host small intestines and enterotoxins to disrupt fluid and electrolyte homeostasis in small intestinal epithelial cells that leads to fluid hyper-secretion and diarrhea. There are 23 CFA or CS (coli surface antigen) adhesins and 2 very distinctive enterotoxins (heat-labile toxin, LT, and heat-stable toxin type Ib, STa) characterized from ETEC strains associated with human diarrhea. As ETEC strains expressing any one or two of these adhesins with either enterotoxin can cause diarrhea, developing vaccines to effectively protect against ETEC diarrhea continues to be challenging.
It has been observed that immunity induced by individual antigens lacked in cross protection against ETEC, clearly due to immunological heterogeneity among CFAs and toxins. Early experimental vaccine studies showed that candidates carrying a single adhesin and/or toxin antigen induced immunity protecting against only ETEC strains expressing the homologous adhesin or toxin. The first ETEC vaccine candidate, killed ETEC prototype strain H10407 (078:H11, LT+/STa+/CFAI+), induced anti-CFA/I and anti-LT immunity and protected against homologous challenge. Similarly to homologous protection from anti-CFA immunity, anti-LT antitoxin immunity protected against ETEC strains expressing the LT toxin but not against ETEC strains expressing STa toxin. Oral whole-cell ETEC vaccine candidates currently under development include rCTB-CF and ACE527. The rCTB-CF is a killed cocktail product of 5 strains expressing 6 CFA adhesins and recombinant B subunit of cholera toxin (CT) which is a homologue of LT. ACE527 carried 3 live attenuated E. coli strains that express 5 CFA adhesins, 1 CFA adhesin subunit, and LTB subunit. The killed rCTB-CF induced antibody responses protecting against 70% ETEC infection or against moderate to severe diarrhea to adults from developed countries travelling to ETEC endemic countries. This product, however, caused adverse effects and provided no significant protection against ETEC diarrhea when given to children especially very young children living in endemic areas, or failed to reduce overall diarrhea incidences among adult travelers. The live attenuated ACE527 initially showed some adverse effects, but these adverse effects were reduced or eliminated when a lower dose was used. ACE527 was shown to induce antibody responses to LTB, CFA/I, CS3, and CS6 among adult volunteers and to protect against severity of diarrhea outcome from homologous challenge.
Although efforts are taken to improve both products, these two candidate products were not optimal in providing satisfactory protection against ETEC diarrhea. Both cocktail products carry no STa antigens to induce antibody response against STa toxin, and require a relatively high dose for oral administration in order to induce host immune responses against each target adhesin and LT toxin. A high administration dose tends to carry excessive somatic antigens, particularly lipopolysaccharide (LPS), which likely cause vomiting among very young vaccine receipients and may mask stimulation of host immune responses specifically to adhesins and toxin. The inability to induce anti-STa antibody response apparently is another cause of lacking in effective protection, as anti-LTB (or anti-CTB) immunity induced by either product may protect only against strains expressing LT toxin but not against STa+ ETEC strains. STa+ ETEC strains are associated with over two thirds of human ETEC diarrhea cases and moderate to severe ETEC diarrhea cases, and are also a leading cause of diarrhea to children younger than 3 years who live in developing countries. Therefore, ETEC vaccines need also to induce protective anti-STa immunity in order to provide effective protection against ETEC diarrhea.
However, the 19 amino-acid STa is poorly immunogenic and potently toxic; thus itself cannot induce anti-STa immunity, nor could it be a safe antigen even if it were immunogenic.
Therefore, there still exists an unmet need to produce a vaccine which can elicit an immune response and can also induce protective immunity to multiple CFA antigens, as well as anti-LT and anti-STa immunity that would be broadly protective against ETEC.